Conventional techniques for calculating a curve road curvature radius of a road that is going to be traveled by a vehicle are disclosed, for example, in the following documents. For instance in, Japanese Patent No. 4,596,063 (JP '063) describes a system having two CCD cameras disposed on right and left front parts of a vehicle and a stereo-image processor. The stereo-image processor captures a field image and objects in front of the vehicle by the CCD cameras in “stereo,” and processes captured image signals of the stereo captured image by using a well-known method for determining a distance, producing a three-dimensional distance image that is, as a whole, composed as a three-dimensional distance distribution, for the purpose of recognizing a three-dimensional road shape and the like.
Further, for example, Japanese Patent No. 3,432,881 (JP '881) describes a steering angle control technique, which controls a steering angle of a steering wheel in a vehicle based on a calculated curve road curvature radius of a to-be-traveled road. Per JP '881, the curvature radius of a curve is calculated based on map information that is a collection of coordinates of many nodes and is output from a map information output unit of, for example, an IC card, a CD-ROM or the like. Based on the calculated curvature radius, the steering angle is controlled.
Further, in Japanese Patent No. 3,720,117 (JP '117), a disclosed technique is used to control a steering angle of a vehicle, based on a calculation of the curvature radius of the curved road by using (i) a distance to a road border, a built-in road facility or the like in front of a subject vehicle detected by a radar and (ii) a distance to such road border/facility from offset positions on the right and left of the subject vehicle.
However, the technique in JP '063 requires two CCD cameras as well as the stereo image processor, leading to a problem that the apparatus/system configuration is complex and the processing is complicated.
Further, the technique in JP '881 cannot always calculate the curve road curvature radius in an accurate manner, due to the lack of accuracy of the map information that is output from the map information output unit. That is, the steering angle control may possibly be inappropriate for the travel of the curve road according to the technique of JP '881.
Furthermore, the steering angle control in JP '117 may also be inappropriate when the road border/facility does not exist, or when a road border line is not recognized by a radar, which disables a calculation of the curve road curvature radius.